Move Guardian reviewer startup into the pool (#45521)

## What changed

Replace `ReviewerSessionFactory` with a startup callback installed through
`ReviewerPool::new`. Review requests supply shared setup data and reuse context,
while the pool uses its callback to create both reusable and forked reviewers.

Update Guardian session setup, prewarming, and existing test fixtures to use the
new pool API.

GitOrigin-RevId: fa171503afcee8bdbdf6822573bea8ded50ce117

codex-rs/core/src/session/mod.rs

@@ -2247,9 +2247,11 @@ impl Session {
 
     /// Uses the extension-owned reviewer, or the same manager for standalone hosts.
     pub(crate) fn guardian_review_session(&self) -> Arc<GuardianReviewSessionManager> {
-        self.services
-            .thread_extension_data
-            .get_or_init(GuardianReviewSessionManager::default)
+        self.services.thread_extension_data.get_or_init(|| {
+            GuardianReviewSessionManager::new(|context, key, kind, snapshot, cancel| {
+                Box::pin(async move { context.spawn(key, kind, snapshot, cancel).await })
+            })
+        })
     }
 
     pub(crate) async fn emit_turn_started(&self, turn_context: &TurnContext) {